Support single-window mode

[williamchange]

Adds support for single-window mode

This may be useful for Linux users where multi-window mode can be unstable(at least until godot resolves the problem),

• i.e. Linux build freezes/crashes when opening external/floating tabs or tooltips #1154

This PR

• Respects --single-window argument via command line
• Adds option in preferences to enable single-window mode (defaults to disabled)
• Adds theme styles(embedded border) for single-window mode
• Fixes for window/popup scale/positions under single-window mode
• Remove unnecessary context menu popup position fixes

[Funkschy]

This would be amazing to have. On wayland the application frequently freezes permanently when overlays/tooltips are on opened